Originally Posted by pink dragon
"So" is used here to mean "in the same way" or "in a similar way". When "so" is used in this way with "be, have and do" or an auxiliary, the subject changes its place. The subject and the auxiliary are inverted, or the subject is inverted with a form of "be, have or do".
He couldn't keep the tunnel's heavy posts in place, and he couldn't resist this one strangely persistent thought. In both cases he was "unable".
Neil left just after midnight and so did Roz.
In the same way; likewise: You were on time and so was I.